Recently, attention is focused on the laminated polyester films having a dye-incorporated intermediate layer(s), for pasting on car windows. In such laminated films, there is a problem that the casting rolls, longitudinal stretching rolls and tenters during manufacture of the films are contaminated due to the sublimation property of the dye even if a dye with high heat resistance is selected.
In Japanese Patent Application Laid-Open (KOKAI) No. 8-230126, in order to solve the above problem, the present inventor has proposed an oriented laminated polyester film for pasting on car window, which polyester film comprises at least three layers comprising two outermost layers containing no dye and an intermediate layer containing a dye, the film satisfying the following three formulae 1 to 3 simultaneously.
0.02 .ltoreq. IB-IA .ltoreq. 0.10 1 0.05 .ltoreq. dA/dB .ltoreq. 0.45 2 0.05 .ltoreq. TA/TB .ltoreq. 0.60 3
In the above formulae, IA represents a viscosity of the polyester composition forming the intermediate layer, IB represents a viscosity of the polyester composition forming both outermost layers, dA a represents thickness (.mu.m) of the intermediate layer, dB a represents thickness (.mu.m) of the outermost layers, TA represents a visible light transmittance (%) of the intermediate layer, and TB represents a visible light transmittance (%) of the outermost layers.)
As seen from a film thickness of 10.5 .mu.m/4 .mu.m/10.5 .mu.m shown in the Examples of the above Japanese patent application, the thickness of the intermediate layer of the said laminated film is smaller than the thickness of the outermost layers in due to the definition of the above formula 2. This thickness relation is based on the reason that sublimation of the dye can be prevented by sandwiching the dye-containing intermediate layer between the relatively thick outermost layers.
However, the laminated film based on the above conception, that is, the intermediate layer being smaller in thickness than the outermost layers, involves the following problem. Namely, the dye concentration necessary for obtaining a film of high light screening effect is increased, and consequently, the film haze is elevated because of clouding caused by precipitation of the undissolved dye.
As for the method of pasting (sticking) the said laminated film on curved car window glass, the conventional method comprising pasting several strips of laminated film is being superseded by the one-piece pasting method which is advantageous in cost and appearance. In this case, it is required to prevent wrinkling of the film when it is pasted on a car window. Also, the laminated film is required to have excellent transparency.
However, the laminated film proposed in the said Japanese patent application is not improved in wrinkle resistance and transparency.